1. Field of the Invention
The present invention relates to a hermetic sealing glass composition for a metal having a thermal expansion coefficient of a range of from 40.times.10.sup.7 to 60.times.10.sup.7 /.degree. C. such as a kovar metal and a hermetic sealing process using the composition.
2. Discussion of Background
An Fe--Ni--Co alloy which is generally called kovar has a composition of 27-28% of Ni, 18-19% of Co and the rest of Fe and a thermal expansion coefficient of about 50.times.10.sup.7 /.degree.C. It is used as a material for glass-sealed wires and packages.
Heretofore, both amorphous glass and crystalline glass are known for hermetic sealing of kovar. However, the amorphous glass has such a problem that it is insufficient in mechanical and thermal strength. As the crystalline glass, Japanese Examined Patent Publication No. 35045/1974 discloses a glass comprising from 9 to 21 mol % of SiO.sub.2, from 14 to 36 mol % of B.sub.2 O.sub.3, from 0 to 16 mol % of Al.sub.2 O.sub.3, from 9 to 56 mol % of CaO+MgO+SrO+BaO and from 9 to 51 mol % of ZnO+CdO. However, it has such problems that the chemical durability and the wetting property to kovar are poor and the yield is low.